The ability to request bandwidth on demand cost effectively has been an area of study for several years. Services like ATM (asynchronous-transfer-mode) SVC (switch-virtual-circuit) allow customers to buy bandwidth at fixed bandwidth increments. With these services, users can purchase the ability to transmit and receive at some maximum bandwidth, and users are allowed to burst (i.e., temporarily increase transmission and/or reception rates) to the maximum port speed they purchase. The performance at burst rate, however, is not guaranteed because the network is allowed to discard cells above the purchased sustained rate under conditions of high network load. This is a harmful drawback because in many applications, service suppliers need to guarantee service performance. For example, a service supplier may, for a certain application, require a 5 second response time but be unable to guarantee that level of performance.
The challenge of providing guaranteed response time is particularly difficult with applications that send and/or receive images because the requirements for bandwidth can vary in these applications from as low as 64 kilobits per second (“kbps”) to as high as 155 million bits per second (“mbps”) within the same imaging application. Table 1 shows typical uncompressed and compressed image sizes and image display times with a 56 kbps transport pipe (i.e., 56 kbps channel bandwidth). Response times can vary dramatically depending upon the size of the image.
TABLE 1Example(time @ImageUncompressedCompressedResponse56 kbps)DimensionSizeSize (JPEG)TimeTypical400 × 300 × 16240kbytes24kbytes3.5secimage640 × 480screenFull page2400 × 3150 × 1615Mbytes1.5Mbytes3.5mincolorimageHigh 10K × 10K × 16200Mbytes20Mbytes48minresolutionphoto-graph
Various ways exist in the prior art to avoid the need for dynamic bandwidth. One of the more widely utilized ways is called Flashpix, developed by Kodak and Hewlett Packard. Flashpix is an image file format that stores images in a square-tile hierarchical file format. Imaging Internet Protocols defined by Hewlett Packard and Kodak, and widely used in the industry, allow application developers to display a defined number of the square tiles at any time in a window at any level within the hierarchy. The applications display only a fixed number of tiles which consume a fixed amount of bandwidth. If a user desires to display the entire image there is no way in the prior art to adjust the bandwidth dynamically to allow the whole image to be downloaded within a guaranteed response time.
For example, Table 2 contains a typical pricing for SVC service. Users are required to pay for a port speed, for example a 45 mbps port, and the per minutes charges shown in Table 2. For a screen image from Table 1, an end user could select a 64 kbps SVC at a variable bit rate of $0.03 per minute. The 24 kbytes screen image would download in 3.5 seconds. If during the session the user wanted to print the 1.5 Mbyte image, current ATM technology allows the application to burst up to the full port speed of 45 mbps for no additional charge. If the network is not congested, the bits will flow at 45 mbps and the 1.5 Mbyte image will be downloaded in less than 3.5 seconds. But if the network is congested, the ATM service will disregard the bits above the contracted 64 kbps SVC rate and the end-user response time is not guaranteed.
TABLE 2BANDWIDTHVARIABLE BITCONSTANT BIT(kbps)RATE ($/min.)RATE ($/min.)64$0.03$0.051,024$0.69$1.152,048$0.95$1.6110,240$10.00 $17.00 
Thus, for many applications, the inability to adjust bandwidth dynamically to guarantee performance is a severe disadvantage.